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relativity principle

Basic principle of special relativity: for two observers moving relative to each other with constant relative velocity (more specifically: for two inertial observers) the laws of physics are the same. There is no key experiment by which one could argue that one of the observers is "at rest" in an absolute sense - as far as physics is concerned, all such observers are equal, no one has more right than another to regard himself as being at rest, and motion (at least: motion with constant velocity) is defined only as relative motion of observers with respect to each other.

For an introduction to some of the consequences Einstein derived from the relativity principle, check out the chapter special relativity of Elementary Einstein.